1. Field of the Invention
The present invention relates to a smoke detecting apparatus which judges a fire by optically detecting smoke particles suspended in air sucked from a monitored zone, by means of laser light.
2. Description of the Related Art
Conventionally, a supersensitive smoke detecting apparatus is used in a computer room, a semiconductor manufacturing facility, or the like. In such a smoke detecting apparatus, the number of airborne particles (aerosol) including smoke particles suspended in air sucked from a monitored zone is optically detected by using laser light, and, when the particle number per unit time period exceeds a predetermined value, it is judged that a fire occurs.
FIG. 8 shows an conventional apparatus. Such an apparatus is disclosed in, for example, Examined Japanese patent publication (Kokoku) No. HEI 5-23709. In the apparatus, laser light emitted from a laser diode 100 is converted into parallel beams 102 by a lens 101, and a smoke detecting area 103 through which sucked air passes is irradiated with the parallel beams. A light receiving device 105 is disposed in a direction perpendicular to an optical axis 104 of the parallel beams 102 which pass through the smoke detecting area 103, receives scattered light due to smoke particles passing through the smoke detecting area 103, and outputs a light reception pulse.
The light intensity distribution of the parallel beams passing through the smoke detecting area has an oval pattern 106 as shown in FIG. 9. It is known that the light emission pattern indicating the light intensity distribution in a sectional direction of laser light emitted from the laser diode 100 spreads in the form of an oval pattern. Also in the case where laser light is converted into the parallel beams 102, therefore, the light intensity distribution has the oval pattern 106 as shown in FIG. 7.
With respect to the light intensity distribution of the parallel beams 102 forming the oval pattern 106, in the prior art apparatus, the light receiving device 105 is disposed in a direction perpendicular to the major axis 107 of the oval pattern 106, thereby allowing scattered light due to smoke particles to be efficiently received.
Recently, as a result of advancement of the improvement of a laser diode, also a laser diode having a light emission pattern which is similar to a true circular pattern wherein the pattern of the major axis direction is identical with that of the minor axis direction has been developed. When a collimator lens is used as the lens 101 for converting laser light into the parallel beams 102, furthermore, diffuse light of an oval pattern from the laser diode can be efficiently converted into parallel beams of a true circular pattern.
Therefore, the parallel beams 102 passing through the smoke detecting area 103 as shown in FIG. 8 becomes closer to a substantially true circular pattern. As a result, the technique of the prior art in which the light receiving device is disposed in a direction perpendicular to the major axis 107 of the oval pattern 106 cannot be applied, thereby producing a problem in that the position where the light receiving device can receive scattered light with the highest efficiency cannot be determined.